This application relates to electrochemical assay devices in the form of single use test strips for detecting the presence or amount of an analyte in a sample and to methods of making and using such devices.
Single use disposable test strips for the electrochemical detection of analytes such as glucose are known. In such test strips, a sample is introduced into the test strip to contact at least two electrodes. Oxidation or reduction of the analyte is observed as a current generated between the two electrodes. Using glucose detection in a conduction cell as an example, as illustrated in FIG. 1, glucose is oxidized by the enzyme glucose oxidase to form gluconolactone and reduced enzyme. The oxidized form of the enzyme is regenerated by reaction with an oxidized mediator with the resulting generation of reduced mediator. This reduced mediator transfers an electron to one electrode, while at the other electrode electrons are transferred onto oxidized mediator, thus producing an observable current. FIG. 2 shows the observable current as a function of time in a test strip using an enzyme/mediator reagent system. In the figure, t=0 is the time of sample application. As shown, the current rises through a maximum, and then declines to reach an eventual steady state plateau. Measurements to determine the amount of analyte are taken after the maximum current has been reached, and generally at a time after the steady state has been achieved.
Before the maximum is reached in FIG. 2, there is a delay observed which can constitute a significant portion of the overall measurement time. The duration of this delay is dependent on the distance between the electrodes, and on the mobility of the mediator employed in the test strip. Mediator mobility is a property of the mediator itself, i.e., the diffusion coefficient, but is also dependent on other sample properties such as hematocrit and viscosity.
In order to increase user convenience, improvements to analyte test strips generally, and to glucose test strips in particular have focused on two major goals: shorter test times and smaller sample volumes. To some extent, these two goals have been achieved in tandem, since smaller sample volumes use smaller cells with smaller electrode-spacing, and smaller electrode-spacing results in shorter reaction time. These cells still have the current/time profile of FIG. 2, however, and thus built-in delay before a measurement can be taken. The present invention eliminates this delay, and thus provides a significant reduction in the time required to complete a test.